Typically, modern furnaces and boilers include a temperature sensor; often called a limit switch, to detect when components of the HVAC system are operating above a threshold temperature. Operation above the threshold temperature could pose a hazard to both the HVAC system and the space that the HVAC system is located within. Generally, when a limit switch opens during an over-heat condition, the control board within the HVAC unit shuts off the fuel supply and power to the furnace, and operates the blower to cool off the air passageway. Also, the system controller (e.g. thermostat), may go blank due to the loss of power. This may continue several times, during which point HVAC unit may operate LEDs on the appliance control board to signal a fault condition, without any indication to the system controller, which is most visible to the user.
Accordingly, there exists a need for an improved system and method to determine a limit fault in an HVAC unit.